This invention relates to a method for improving the antireflective properties of materials by application of a surface grating. By the use of multiple stair-steps in the grating, and/or by the use of crossed gratings, and/or by the use of dielectric overlayers the material can be made non-reflective to a wide range of frequencies, a wide range of angles of incidence and for any polarization or combination of polarizations. The method involves a consistent mathematically rigorous procedure for the selection of appropriate grating periods and filling factors to be applied to the surface by holography, electron beam lithography, reactive ion etching or other techniques. It is applicable to a broad frequency range including optical and radar frequencies as well as any arbitrary angle of incidence. The antireflective treatment can be applied to dielectric, semiconductor, or conductor materials.
Thin film antireflection coatings are often used to produce low reflectivity at an interface with a dielectric material. Thin films require deposition of dissimilar materials and display problems such as differential expansion and contraction, adhesion, and local heat absorption. Thin films degrade at high power and have a limited range of indices of refraction, the minimum being about 1.35. Alternatively, a high spatial-frequency periodic surface-relief grating on a dielectric surface can also behave like an antireflection grating.
In the prior art, the patent to Moshinsky, U.S. Pat. No. 4,501,784 discloses the use of closely spaced indentations in a random or systematic pattern having a depth or radius of curvature from 0.2 to 5.0 times the wavelength of the radar signals which may illuminate the surface. The indentations vary in depth or radius of curvature within the stated range and are variably positioned on the surface. The surface irregularities are spaced in size so that they will scatter or disperse any of the various radar wavelengths on reflection. The surface irregularities may be distributed uniformly or randomly over the surface. The effect is to reflect radar signals in a highly dispersed manner so that the radar signal returned to the radar apparatus has an intensity less than 1 percent of the signal reflected from a smooth surface.
The patent to Akiba et al., U.S. Pat. No. 4,660,934 discloses a method for manufacturing a diffraction grating having periodic corrugations through two-beam interference exposure so that the phases of corrugation in two adjacent regions are reverse to each other. The method involves first forming a negative type photoresist film on a region of a substrate and then forming a positive type photoresist film in a second region of the substrate. The second step involves subjecting the two regions of the substrate to two-beam interference exposure and in the third step, through utilization of characteristics of the negative and positive photoresist films, a diffraction grating is formed in which corrugations in the first region and second region are reversed in phase to each other.
The patent to Southwell, U.S. Pat. No. 4,666,250, discloses a multilayer antireflective optical film which, when placed between an incident medium and a substrate effects minimal reflectivity from the instrument medium-substrate interface over a broad spectral band. The optical film is comprised of a plurality of thin layers of equal thickness each of which is either a selected first material having a low refractive index or a selected second material having a high refractive index. The low or high refractive index is specified for each layer, then the refractive index for a single layer is changed and the reflectivity reevaluated. Each layer is examined in turn until no further improvement in reflectivity is obtained.
The method of this invention involves first the calculation of the required thickness and complex refractive index of a single homogeneous layer on lossy substrate to produce zero reflectivity using a rigorous impedance matching approach. The method is applicable to both transverse electric (TE) and transverse magnetic (TM) polarization, and to any angle of incidence. The method then involves the calculation of the required filling factor and groove depth of a rectangular-groove grating that is equivalent to that of the single homogeneous layer in the long-wavelength limit.